Pedestal-type office chairs have conventionally utilized a tilt-type control mechanism connecting the upper end of the pedestal to the chair seat. This control mechanism defines a substantially horizontal tilt axis which extends sidewardly across the chair directly adjacent the underside thereof, with the tilt axis being disposed substantially directly over the pedestal and hence disposed substantially midway between the front and rear edges of the chair seat. With this mechanism, rearward tilting of the chair seat results in the rear edge of the seat swinging downwardly, and simultaneously the front edge of the chair seat lifts upwardly causing undesired lifting of the occupant's legs in the vicinity of the knees. Tilt control mechanism of this type have long possessed this recognized disadvantage, but have nevertheless been extensively utilized in view of the difficulties in resolving this problem.
In recent years chair manufacturers have succeeded in developing a knee-tilt control mechanism. This mechanism again connects to the upper end of the pedestal but is positioned forwardly therefrom, whereby the sidewardly extending horizontal tilt axis is hence disclosed more closely adjacent the front edge of the chair seat. In this manner, rearward tilting of the seat structure is accomplished solely by a downward tilting of the rear edge of the seat, with the front edge of the seat experiencing only minimal elevational change. The occupant can thus experience tilting of the seat structure without encountering undesired lifting of the legs away from the floor.
The design of a proper knee-tilt mechanism has presented several formidable problems since such mechanism has to be cantilevered forwardly from the upper end of the pedestal, and at the same time the mechanism must be disposed within a package which does not ruin the appearance of the chair.
Most attempts to provide a knee-tilt mechanism have employed a spring-type restoring device using torsion or compression springs, the latter cooperating with levers or a linkage for continually urging the seat structure upwardly into its normal horizontal position when unoccupied. These spring-type restoring devices have, for the most part, created a restoring force which substantially linearly increases as the tilt angle increases, the latter typically being a minimum of about 16.degree. downwardly from the horizontal. Because of the substantially linear relationship of the restoring force, the known mechanism have possessed disadvantages which have made use of these mechanisms, and the use and comfort of the chairs employing them, less than desired.
For example, the known knee-tilt mechanisms have normally employed a substantially linear restoring spring arrangement which possesses a spring rate such that the restoring force increases significantly as the seat structure is tilted backwardly. This significant increase in the spring force is required so as to support the chair occupant and counterbalance the backward tilt. If a low initial torque and low spring rate are used, it has been observed that when the occupant initially sits in the chair, the weight of the occupant itself causes the seat structure to tilt backwardly through a substantial extent, such as up to about 10.degree.. This has been observed to be an undesirable degree of tilt for supporting the occupant, since a rearward tilt in the range of 3.degree. to 5.degree. is preferred under such circumstance. This larger tilt is undesirable since it detracts from the chair comfort when working at a desk or table.
To overcome this latter problem, several different structures have been tried. The primary attempt has involved the use of a mechanical lock which is manually controlled by the chair occupant. That is, the knee-tilt mechanism is maintained with a spring mechanism having properties of the type explained above, and in addition the mechanism is provided with a manually controlled mechanical lock. This lock is normally activated by the occupant and, in effect, results in the chair seat being fixed in its upright position, that is, the seat being oriented substantially horizontally. When tilting of the seat is desired, the occupant has to release the mechanical lock so that the tilt mechanism then permits rearward tilting of the chair seat. Needless to say, the provision of this mechanical lock greatly detracts from the comfort and flexibility of the chair since the occupant must basically always be converting the chair from a fixed to a tilt condition, or vice versa, and this manual manipulation obviously detracts from the desirability of the chair.
In other attempts to overcome this problem, other variations of the tilt mechanism have used a higher spring rate, and/or have increased the initial restoring force (i.e., the precompression or pretorque) of the spring which maintains the unoccupied seat structure in its horizontal position. Increasing the spring rate and/or initial restoring force thus tends to counteract the initial weight of the occupant. These changes, however, also cause the linear relationship of the restoring force to be increased or shifted upwardly throughout the complete tilt range so that, when a person attempts to tilt the chair seat backwardly throughout substantially its full range, it has been observed that many occupants are unable to exert (at least comfortably) sufficient force so as to permit rearward tilting of the chair throughout substantially the full tilt angle. Under this circumstance, the chair occupant again finds the chair highly uncomfortable due to the inability to comfortably tilt backwardly the full extent, and due to the excessively large restoring force which the occupant must overcome.
Accordingly, it is an object of this invention to provide an improved knee-tilt control mechanism for a chair, which mechanism is believed to overcome many of the disadvantages which have been associated with prior structures as explained above. The knee-tilt control mechanism of this invention is particularly of the passive type in that it does not require any change or action by the occupant, but rather permits automatic reclining when desired.
More specifically, it is an object of this invention to provide an improved knee-tilt control mechanism for a chair, which control provides a substantially nonlinear restoring force throughout the angle of tilt so as to provide adequate stiffness and restoring force to maintain the chair seat at a desired position under normal use conditions with an occupant therein, while at the same time permitting the chair to be tilted rearwardly throughout substantially its full range without generating excessively large restoring forces which make tilting difficult or uncomfortable. In the improved knee-tilt control, the nonlinear restoring force preferably includes a linearly increasing restoring force over the initial range of tilt such as from the 0.degree. position to about the 5.degree. position, with the restoring force thereafter being substantially linear but at a lesser rate so as to prevent the maximum restoring force at the full tilt angle from reaching an excessive magnitude.
In the improved mechanism of this invention, as aforesaid, the initial restoring force, and the increasing restoring force as the chair seat tilts rearwardly due to the weight of the occupant, is such as to maintain the chair seat at only a small rearward tilt angle with respect to the horizontal, such as a maximum tilt angle of about 3.degree. to 5.degree., so as to hence maintain an optimum seating position for the occupant. At the same time, rearward tilting of the seat throughout its full range can be easily accomplished, even by a person of rather light weight, without encountering excessive restoring force which makes such tilting uncomfortable or impossible.
A further object is to provide an improved mechanism, as aforesaid, which is relatively compact and hence can be structurally and properly designed so as to be positioned directly under the front portion of the chair seat without detracting from the overall esthetics or appearance of the chair. This improved mechanism also possesses the capability of permitting the initial restoring force or torque to be selectively adjusted without requiring any complex adjustment function or disassembly of the mechanism.
According to the present invention, the seat structure and pedestal of a chair are joined together by a knee-tilt control mechanism which includes a first support which is fixed to and projects downwardly from the underside of the chair seat adjacent the front edge thereof, and a second support which is fixed to the upper end of the pedestal and projects forwardly therefrom so as to terminate in a generally sidewardly extending tube structure. The first support has a pair of bearing hubs at opposite ends thereof which are rotatably engaged with the tube structure so as to define a horizontal tilt axis which extends sidewardly of the chair seat and is disposed closely adjacent the underside thereof in close proximity to the front edge. A spring-type restoring mechanism is disposed substantially within the tube structure and coacts between the first and second support for exerting a restoring moment which continuously urges the chair seat upwardly into a substantially horizontal position. The restoring mechanism employs a pair of compression springs, such as stacks of Belleville springs, disposed within the tube structure on axially opposite sides of an adjustable pre-torque member which can be selectively rotated so as to set the initial compression level of the springs. The outer ends of the springs bear against actuator members which are coupled to and rotate with the bearing hubs of the first support. The actuators have radially outwardly projecting cam followers for engagement with ramp-like cams which are formed on the tubelike structure and extend circumferentially thereof through the permissible tilt angle. When the seat structure is tilted rearwardly causing rotation of the first support and hence corresponding rotation of the actuators and cam followers, the cam followers ride against the cam ramps and cause the actuators to be displaced axially inwardly to increase the compression of the springs in accordance with the profile on the cams. The cams have a nonlinear profile so that the displacement of the actuator and corresponding compression of the springs increase at a first rate throughout the initial tilt angle, such as from 0.degree. to 5.degree. , with the cam ramp then being of lesser slope so that the rate of axial displacement and corresponding rate of spring compression hence decreases throughout the remaining angular tilt of the seat structure.
Other objects and purposes of the invention will be apparent upon reading the following specification and inspecting the accompanying drawings.